Guestbook Spamming

I got a php guestbook which is heavily spammed with messages like

"I like your website. Thanks for the good information and entertainment. Keep up this great resource!"

You done great work. Congratulations!

The messages are mostly similar but they are posted from different IPs. I suspect these messages are created by robots.

Is there anyway to stop this?

Assuming you also grant as linkback... you can simply stop granting the linkback. That should take away their motive.